Frequency mixers are critical components in RF and microwave systems that convert a signal from one frequency to another. FIG. 1 shows a conventional switching/commutating mixer 10 also known as a double balanced mixer. It is also called a passive mixer because there is no DC current flowing through FET devices in the mixer.
The mixer 10 provides frequency up-conversion and down-conversion by converting the frequency of a signal applied at its differential input nodes input+ and input− into the frequency of a signal produced at its differential output nodes output+ and output−. Local oscillator (LO) nodes LO+ and LO− are supplied with signals LO+ and LO− at the LO frequency to control up-conversion or down-conversion of the input frequency. For up-conversion application, frequency components of the signal at the output nodes include the desired frequency of the input signal plus the LO frequency. For down-conversion application, frequency components of the output signal include the desired frequency of the input signal minus the LO frequency or the LO frequency minus the input frequency.
The switching/commutating mixer 10 includes MOSFETs M1, M2, M3 and M4 controlled by LO signals at the nodes LO+ and LO−. The LO signals controls the MOSFETs M1 to M4 to switch or commutate the input signals at the LO rate so as to produce the output signal at a frequency which is the difference or sum of the mixer's input frequency and the LO frequency.
The most important parameters defining a mixer's performance are conversion gain (Gc), input third order intercept point (IIP3) and noise figure (NF). The conversion gain is a ratio of an output power of the mixer to an input power of the mixer. The third order intercept point is a point at which the power in the third-order product and the fundamental tone intersect, when the mixer performance is assumed to be linear with respect to power input. Hence, the third order intercept point quantifies the non-linearity of the mixer. The noise figure is a measure of degradation of the signal-to-noise ratio (SNR), caused by components in a radio frequency (RF) signal chain.
Generally, the mixer 10 may have conversion loss of about 6 dB or more. To add gain, a mixer system needs an amplifier stage that can be coupled after the mixer 10. The amplifier and the mixer can be arranged as two separate chips or the amplifier can be integrated onto the chip of the mixer. In case of two chips, both chips are usually designed to be 50 ohm single ended at their input and output ports. In the integrated solution, a common emitter amplifier can be used, and the input impedance of the amplifier should be made about 50 ohm so that the input impedance of the mixer can be easily converted to 50 ohm single-ended impedance. In both cases, the IIP3 of the mixer system will be limited because the impedance between the output of the mixer 10 and the amplifier input is about 50 ohm. Also the amplifier stage will cause a pole which results in limited output bandwidth.
Therefore, there is a need for a new mixer topology that would improve linearity and conversion gain of a conventional mixer, while providing a low noise figure.